1. Field of the Invention
The present invention relates to an optical scanner in an image forming apparatus.
2. Description of the Related Art
Conventionally, as an image forming apparatus that forms an image by using the Carlson process, for example, the one that forms a latent image on a surface of a rotating photosensitive drum by scanning optical beams via a polygon mirror, and fixes a toner image obtained by visualizing the latent image on a sheet of paper as a recording medium, thereby forming an image has been known. In recent years, this type of image forming apparatus has been frequently used for simple printing as an on-demand printing system, and therefore demands for higher densification of the image and speed-up of image output are increasing.
One approach to speed up image output, is to increase the rotation speed of the polygon mirror that deflects the optical beams and rotation speed of the photosensitive drum. However, with an increase in the rotation speed of the polygon mirror, noise and vibrations from a drive system and power consumption increase, thereby deteriorating overall durability of the apparatus. Speed-up of image output has a trade-off relationship with densification of the image. In other words, with an increase in the rotation speed of the polygon mirror, the image quality degrades.
Therefore, as a method of achieving both the higher densification of the image and higher image-output speed, such a method has been proposed that a light source emits multi-beams to scan the photosensitive drum with a plurality of optical beams at a time (for example, see Japanese Patent Application Laid-open No. 2005-250319 and Japanese Patent Application Laid-open No. 2004-287292). The methods described in these patent documents collectively deflect diverging rays from a vertical-cavity surface-emitting laser (VCSEL) having a plurality of luminous points by the polygon mirror, in a state with the diverging rays being coupled by a coupling lens, thereby enabling to scan the photosensitive drum simultaneously with the optical beams.
To emit multi-beams from the light source, for example, a method that employs a plurality of edge emitting type laser diodes (LDs) as the light source and a method that employs one-dimensional or two-dimensional edge-emitting type laser array as the light source can be considered. When the edge emitting type LD is used as the light source, a general-purpose LD (LD that emits only one beam) can be used, thereby enabling production cost reduction. However, it is difficult to stably maintain the positions of the LD and the coupling lens between a plurality of LDs, and nonuniformity of scanning line interval is likely to occur. Accordingly, it is difficult to achieve higher densification of the image. Further, because the number of LDs that can be arranged receives a constraint, it is difficult to simultaneously achieve the higher densification and the higher image-output speed.
Further, when the edge-emitting type laser array in which the luminous points are arranged one-dimensionally is used, the scanning line interval by the optical beams can be made uniform. However, the power consumption by the light source increases. When the number of beams is increased too much, a deviation amount of the optical beams from the optical axis of optical elements constituting an optical system increases, thereby causing a problem such that optical characteristics of the apparatus deteriorate.
On the other hand, because the vertical-cavity surface-emitting laser emits the optical beams vertically with respect to a substrate, two-dimensional integration of the luminous points is easy, and the power consumption can be reduced by one digit as compared to the edge emitting type element. Accordingly, it is advantageous in realizing super densification of the image and speed-up of image output.
However, there is a major problem in the vertical-cavity surface-emitting laser in that it is difficult to achieve high output of the optical beams due to a small volume of an active layer. For example, the output of the edge emitting type semiconductor laser generally used in an optical scanner in the image forming apparatus is about 7 milliwatts to 10 milliwatts, whereas the output of the current vertical-cavity surface-emitting laser is about 1 milliwatt to 2 milliwatts. Therefore, to realize higher image-output speed using the vertical-cavity surface-emitting laser, the optical beams need to be used more effectively as compared to a case that the edge emitting type light source is used.
Specifically, the semiconductor lasers that include the vertical-cavity surface-emitting laser has a variation of the divergence angle for each laser element. The variation causes a change in effective numerical aperture (NA) in the optical system in the optical scanner, and also causes a variation of beam spot diameter and an increase of the beam spot diameter on the surface to be scanned. To avoid these problems, a beam width of the beams emitted from the laser element to be guided onto an image surface is shaped by using an aperture member. However, if the aperture member is used, light use efficiency (quantity of light reaching the image surface out of the quantity of light emitted from the light source) decreases. Thus, there is a need to improve the light use efficiency when the vertical-cavity surface-emitting laser is used as the light source.